Bones comprise an organic component (cells and matrix), and an inorganic or mineral component. The cells and matrix comprise a framework of collagenous fibers which is impregnated with the mineral component, chiefly calcium phosphate (85%) and calcium carbonate (10%), which imparts the quality of rigidity to bone. In a disease commonly known as osteoporosis, the bone demineralizes and becomes abnormally rarefied. While osteoporosis in general predominantly afflicts the elderly, a specific form of osteoporosis known as disuse or immobilization osteoporosis has been identified with immobilized persons of all ages whose bones are not subject to functional stress. In these cases, some patients will experience significant loss of cortical and cancellous bone during prolonged periods of immobilization. Quite often, an elderly patient immobilized after a fracture of a long bone may experience bone loss due to disuse, which may ultimately lead to secondary fractures in an already osteoporotic skeleton. Diminished bone density may lead to verterbrae collapse, fractures of the hips, lower arms, wrists, ankles and incapacitating pain.
Attempts at treating and/or preventing osteoporosis by means of electrical signals include the work of McElhaney and Stalnaker who in 1967 reported that a 200 V and 30 Hz field applied between two parallel plates to a cast-immobilized femur of the rat could reduce the bone weight loss and bone cortical area reduction associated with immobilization. However the number of animals was too small to yield results of statistical significance.
Experiments using signals of the same frequency were repeated in 1978 by Martin and Gutman who found statistically significant increases in the weight (ash weight) and in the cross-sectional area of stimulated femora on one leg of a rat relative to unstimulated femora on the other leg. Their experiments utilized parallel plates applied to the cast-immobilized hind limb of rats, with an electrical signal of 100 v/cm at 30 Hz applied to the plates for periods of 2 to 8 hours per day. Some positive results were obtained, and an increase in stimulation times was shown to improve the results.
Kenner in 1975 showed that Pt-10% Rh electrodes implanted one centimeter apart in the heel of an eight week old male New Zealand rabbit whose legs were immobilized by casting could prevent bone loss due to disuse when measured by graded histologic bone sectors. These results were observed using both constant DC and pulsed DC stimulation signals. Further, Bassett showed that an inductively coupled electric field could enhance the mechanical strength of embedded rat tibia as measured by compression in a plunger load-cell apparatus.
Thus, prior work at treating and/or preventing osteoporosis has included the use of relatively high amplitude voltage signals, necessitating high power sources. Further, the stimulation signals used have been limited to constant or pulsed DC signals, or AC signals of relatively low frequency. Lastly, the results of this prior work has in some cases been inconclusive and discouraging.